Electrolytic capacitor with improved seal

ABSTRACT

An electrolytic capacitor is provided with improved sealing means comprising an adhesive seal between the primary, elastomeric seal and the secondary, hermetic glass-to-metal seal to prevent reaction between the electrolyte and exposed metallic portions adjacent the glass-to-metal seal.

United States Patent Correll [4 1 Aug. 15, 1972 [341 ELECTROLYTICCAPACITOR WITH 3,321,675 5/l967 131 6118 ..311/230 IMPROVED SEAL3,302,012 1/1967 011611 ..311/230 Inventor: Edward M. Corrcll, WestColumbia,

General Electric Company July 12, 1971 Assignee:

Filed:

Appl. No.:

US. Cl ..3l7/230, 29/570 Int. Cl. ..H0lg 9/10 Field of Search.....'..3l7/230, 231, 233

References Cited UNITED STATES PATENTS 4/1966 Griffin et al. ..3l7/2303,534,230 10/1970 Krasienkoetal ..3l7/230 Primary Examiner-James D.Kallam Attorney-Nathan J. Comfeld et al.

[57] ABSTRACT An electrolytic capacitor is provided with improvedsealing means comprising an adhesive seal between the primary,elastomeric seal and the secondary, hermetic glass-to-metal seal toprevent reaction between the electrolyte and exposed metallic portionsadjacent the glass-to-metal seal.

3Clainn,3Drawingl igures PATENTEDAUG 1 5 I972 3684.927

I f R INVENTOR: EDWARD M. CORRELL,

HIS ATTORNEY.

BY W P ELECTROLYTIC CAPACITOR WITH IMPROVED SEAL BACKGROUND OF THEINVENTION This invention relates to electrolytic capacitors. Moreparticularly, the invention relates to hermetically sealed electrolyticcapacitors having glass-to-metal or ceramic-to-metal seals.

Electrolyte leakage from sealed electrolytic capacitors under abnormalconditions, such as, pressure or temperature extremes in the surroundingenvironment have resulted in a number of seal constructions all havingthe common goal of eliminating electrolyte leakage. While this primaryproblem of leakage of the electrolyte from within the can to theexternal surroundings can be solved by the use of glass-to-metal orceramicto-metal sealing techniques, other problems have been encounteredin the course of this solution. Conventional capacitors of this typenormally comprise a sintered slug of an oxide-forming metal, such as,for example, tantalum, having a lead of the same metal incorporatedtherein which becomes the anode lead. The slug is commonly mountedwithin a cylindrical metal case or can which may form the cathode leadfor the material and electrolyte is placed therebetween. Elastomericmaterials are placed around the lead adjacent the open end of the canand the elastomeric materials are expected to form a primary seal toprevent passage of the electrolyte therethrough. Beyond this elastomericseal a glass-to-metal seal or ceramic-to-metal seal is used. Theglass-to-metal seal comprises a center metallic member which may be asolid rod or a hollow tube having a glass bead bonded thereto and ametal sleeve around the glass bead and bonded thereto. The central leador tube is in turn fastened, such as by welding, to the anode lead andthe metal sleeve is then soldered or welded to the case. The resultingglass-to-metal seal provides an hermetic seal through which theelectrolyte cannot pass.

The problem, however, which has been created is that the materials usedfor the glass-to-metal seal, particularly the metal materials may reactwith the electrolyte if a portion of the electrolyte penetrates theprimary, elastomeric, seal. If, for example, the metal tube or rod,bonded to the glass bead and subsequently fastened to the anode lead isnot of a film-forming metal such as tantalum, any electrolyte passingthrough the primary seal may form a shorting bridge between the tube orrod and the case. Even when the tube or rod is made of a film-formingmaterial such as tantalum, a problem may arise if the material has notbeen previously anodized. This is because the electrolyte penetrationforms a temporary high current path while the oxide film is beingformed. This can result in a short spike or current surge which may havea deleterious effect upon the circuitry in which the capacitor is incorporated.

Many improvements on the elastomeric primary seal have been proposed toavoid the penetration of the electrolyte therethrough. Obviously,however, the very presence of the secondary glass-to-metal seal isindicative of the fact that the primary elastomeric seals are notabsolutely electrolyte leakage-proof. Thus, the problem or possibilityof leakage through the primary elastomeric seal must be reckoned anddealt with.

It has been proposed to provide another compression seal around the leadadjacent to the glass to protect the tube from electrolyte attack.Maintaining the compression while attaching the glass-to-metal seal,however, presents additional manufacturing expense. Also, thetemperature extremes previously referred to may provide furthercomplications because the resiliency of the material, upon which thecompression seal depends, may change considerably through thetemperature range to which the device may be exposed resulting in lossof compression and possible failure of the seal.

It is therefore an object of this invention to provide means to protectthe metallic portions of the glass'tometal seals from electrolyteattack. It is a further object of the invention to isolate portions ofthe glass-to-metal seal from the electrolyte by an adhesive sealindependent of the primary elastomeric seal. Other objects of theinvention will be apparent from the disclosure and.

drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view ofthe invention. FIG. 2 is an enlarged, fragmentary view of a portion ofFIG. 1.

FIG. 3 is an alternate embodiment of the invention.

DESCRIPTION OF THE INVENTION Referring now to FIGS. 1 and 2, a capacitoris generally indicated at 2 comprising a wet slug, electrolyticcapacitor which, in the illustrated embodiment, comprises a sinteredtantalum anode 10 which has previously been formed or anodized toprovide a thin dielectric layer of tantalum oxide thereon as thedielectric material. The anode is mounted in a cylindrical case 14having a closed end 14a and an open end 14b. The anode is centrallypositioned therein by a spacer member 16 resting on the closed end 14aof case 14. A tantalum lead 12 is centrally positioned in the anode andextends toward the open end of case 14.

Elastomeric seal means and glass-to-metal seal means are providedadjacent to the open end 14b to seal case 14 as will be described below.A cylindrical bushing 20, having a diameter approximately equal to theID. of case 14 and comprising a relatively inert plastic material, suchas polytetrafluoroethylene, is inserted into open end 14b of case 14above anode 10. Bushing 20 is centrally punctured by lead 12 to form aseal therebetween as will be later described.

Bushing 20 is also formed with an annular groove 22 into which an O-ring26 is placed. Case 14 is subsequently crimped at 30 to press the metalcase into the bushing slightly above the groove to thereby compressO-ring 26 into sealing engagement between bushing 20 and case 14.Bushing 20 is also provided with a central depression 24 which may becone-shaped for a purpose which will be described presently.

Above bushing 20, a glass-to-metal seal is provided comprising a glassbead 50 having a central, metallic, tube 52 bonded thereto and an outer,metallic, sleeve 54 also bonded thereto. The seal may be a matchedcoefficient seal or a compression type of seal. Glass-tometal sealswhich can be used in the invention include, for example, those describedand claimed in Merritt et al. US. Pat. No. 3,275,901 assigned to theassignee of this invention or in my copending application, Ser. No.

889,004. The exact choice of glass material will depend, among otherthings, upon the type of electrolyte. A solderable lead 56 comprising ametal such as nickel or copper is welded at 58 to the tantalum lead 12to form an extension thereof. The position of this weld is slightlybelow the top of tube 52 so that only the solderable lead extension 56of tantalum lead 12 extends beyond the top of tube 52. Theglass-to-metal seal is mounted into the open end 14b of case 14 andsealed thereto by soldering tube 52 to solderable lead 56 and solderingor welding metallic sleeve or ring 54 to case 14 as shown respectivelyat 62 and 64. Thus, the capacitor is completely sealed bymetal-to-metaland metal-to-glass seals. Alternatively, the glass portionmay be replaced by a ceramic member to which the metal portions arebonded by techniques known to those skilled in the art. The use of theterm glass-tometal seal herein is therefore intended to include bothglass and ceramics.

Before the seals are mounted and secured to case 14, it must be filledwith an electrolyte to provide the electrical connection between case 14which forms the cathode lead and the interface of the anodizeddielectric material. In the preferred embodiment, a sulfuric acidelectrolyte is placed within case 14. This electrolyte, as is well knownin the art, also serves to reoxidize or anodize any portion of the oxidedielectric layer which may break down during use. The electrolyte isbasically Y contained within the space surrounding the anode by bushing20 and O-ring 26. Minor amounts of electrolyte, however, may escapebeyond this primary seal, either via the peripheral case wall orcentrally between bushing 20 and lead 12. In either case, the sulfuricacid comes in contact with the metal and glass portions of theglass-to-metal seal. Glasses may be chosen having resistance to sulfuricacid and the amounts of acid which pass through the primary seal areminimal and thus attack of the metal portions of the glass-to-metalseals will not destroy the seal. It is, however, possible that a currentleakage bridge may develop between the case 14 and the metallic tube 52which is in electrical contact with anode 10. This, of course, providesa direct short between the cathode of the capacitor and the anode. Inaccordance with the invention, the formation of such a leakage bridge isprevented by encapsulating the exposed portions of tube 52 adjacent theglass-to-metal seal before the final sealing of case 14 to theglass-to-metal seal.

This is accomplished in accordance with the invention by filling thecavity 24 in bushing 20 with an adhesive material 60 such as a caulkingcompound or a curable, elastomeric material providing reasonableresistance to attack by electrolyte 66, and which will provide anadhesive bond or seal to the tantalum lead 12, the glass bead 50, andmetallic tube 52 to prevent any passage of the electrolyte either viathe periphery of case 14 or from the central opening in bushing 20 toreach tube 52. The terms adhesive bond or adhesive seal as used hereinare intended to define a bond or seal formed by a material which willcontact glass bead 50, tube 52, and lead 12 to form a reasonablyelectrolyteimpermeable seal or bond by an adhesive force rather than acompressive force. Thus, when the sealing material is exposed to wideextremes of temperature, the seal will not be affected since it dependsupon the adhesive bond of the sealing material to the lead 12, the glassbead 50, and the metallic tube 52 rather than compression forces urgingthe sealing material against the aforementioned components. Theadvantages of this sealing mechanism are further evident from the factthat manufacturing tolerances relied upon for a compression seal mayresult in a lack of compression as the compressing member contracts at avery low temperature, while, on the other hand, any contraction of theadhesive seal results in a small space between the adhesive seal and thebushing 20 rather than a rupture of the bond between the sealingmaterial and the glass bead, tube, and lead. This is because the bushing20 is constructed of a relatively inert material such aspolytetrafluoroethylene which will not form an adhesive bond with thesealant material.

The adhesive material preferably comprises a curable caulking materialwhich is placed into cavity 24 before the glass-to-metal seal has beenplaced into open end 14b of case 14. Preferably, cavity 24 is filledlevel with the caulking material. After the caulking material has beenplaced in cavity 24 the glass-to-metal seal is placed into opening 14bof case 14 and glass bead 50 and tube 52 are brought into contact withthe sealing material which is level with the top of cavity 24.Alternatively, if slightly less material is used, the assembly may beinverted after insertion of the glass-to-metal seal to cause some of theadhesive material to flow out of cavity 24 and contact both glass bead50 and tube 52. If the caulking material is a non-curable material whichstays soft and resilient the glass-to-metal seal may then be sealed aspreviously described by soldering lead 56 to tube 52 and ring 54 to case14. If the material is a curable elastomer, the material is allowed tocure before the sealing operation to allow oxygen and moisture topenetrate into the material and assist in the curing.

While, as stated above, the caulking material may be a non-curablesealing material such as a soft, tarry material or other well knowncaulking compound having adhesive properties, preferably the sealingcompound comprises a curable, elastomeric material such as a curable,silicone rubber or a fluorosilicone rubber which will form an adhesivebond to the glass and metal portions of the glass-to-metal seal and tothe lead, yet, after curing, will retain its elastomeric quality therebyproviding additional sealing qualities by virtue of the resiliency ofthe material. A preferred material is a room temperature curablesilicone rubber such as, for example, the General Electric RTV seriesadhesive sealants. Materials of this type are described more fully inKulpa US. Pat. No. 3,296,161 and Beers US. Pat. No. 3,382,205 bothassigned to the assignee of this invention.

Turning now to FIG. 3, an alternate construction is illustrated whereinmetal tube 52 is replaced by an oxidizable or film-forming metal tube 52such as tantalum. In this construction tube 52' and lead 12' are weldedtogether to close tube 52' and the solder joint 62 is replaced by a weld62' which welds or bonds solderable lead 56' to lead 12' and tube 52.Prior to making these welds the caulking material is placed in cavity 24as in the preferred embodiment already described. In this embodiment thestructure of the invention protects tube 52 identically to that of FIGS.1 and 2 but for a slightly different reason. In the embodiment shown inFIGS. 1 and 2 the adhesive material is used to prevent the formation ofa current leakage bridge. In this embodiment, the material is used toprevent the oxidization of the tantalum sleeve while the capacitor is inactive use which oxidization would provide a momentary-butundesirable-surge of current.

To further test the efficiency of the seal of the invention capacitorsusing the seal of the inventions, were constructed similar to FIG. 3 andexposed to extended testing at 85 C and 125 C and the capacitance,dissipation factor, and DC leakage measured periodically to determinewhether these parameters met the requirements of various military typespecifications such as, for example, Mil-C-39006. After 10,000 hours ofcontinuous testing the deviation of the capacitance, dissipation factor,and DC leakage did not exceed the allowable tolerances in the abovespecificanon.

Thus, the invention provides an improved seal structure for anelectrolytic capacitor whereby the effect of any deleterious attack ofthe glass-to-metal seal by the electrolyte is mitigated by the isolationand encapsulation of the exposed metal anode portions of theglassto-metal seal by an adhesive seal which bonds to the glass and tothe metallic portions.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An improved electrolytic capacitor comprising a case having a closedend and an open end, an anode comprising a film-forming metal within thecase and having a lead thereon of the same metal and generallypositioned centrally with respect to said open end of said case, aninsulating resilient sealing means surrounding said lead and contactingthe inner surface of said case, a glass-to-metal seal closing said openend of said case and generally comprising a glass bead having sealedthereto a metallic tube centrally positioned therein and a metallic rim,said rim being sealed to said case and said tube being metallurgicallybonded to said lead to provide an hermetic seal, the improvementcomprising said sealing means having a cavity around said lead, aninsulating adhesive material surrounding said lead in said cavity andforming an adhesive sealing bond with said lead, and adhesively sealingwith said glass bead and metallic tube for preventing electrolyte of thecapacitor from contacting said tube including said metallurgical bond.

2. The capacitor of claim 1 wherein said insulating adhesive materialcomprises a resilient, non-curing caulking compound.

3. The capacitor of claim 1 wherein said insulating adhesive materialcomprises a curable sealant.

2. The capacitor of claim 1 wherein said insulating adhesive materialcomprises a resilient, non-curing caulking compound.
 3. The capacitor ofclaim 1 wherein said insulating adhesive material comprises a curablesealant.